Re: North Pole

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding Michael E.'s description of his disagreement with John D.:

> I think John and I are visualizing the same physics, but we disagree
> on word definitions.
>
> John says the celestial sphere is not polar aligned, and I say it
> is.  John says the stars do not change their positions on the
> celestial sphere.  I say they do.  This boils down to the definition
> of celestial sphere.  I teach astronomy even though I am only
> an amateur astronomer.

Michael then supposedly supports his claim that the celestial sphere
is defined by the celestial poles and the celestial equator with a
quote from an intro astronomy book on his shelf. Part of the quote
reads:

> "Just as we use the earth's poles and equator as reference marks on
> the earth, we can use corresponding reference marks on the sky.
> The celestial poles and celestial equator are defined by the earth's
> rotation.
> <snip>
> In fact, the celestial poles and equator are the basis for a system
> of precise celestial coordinates much like the system of latitude
> and longitude on the earth,  ...."

But this does not define the celestial sphere.  It merely describes
some interesting landmarks on it (that happen to be useful for
giving a particular coordinatization of it). ...

The quote continues:

> <snip>
> "Over centuries precession has dramatic effects.  For example, it
> makes the celestial poles move across the sky.  ... The pole is
> now approaching Polaris and will be closest to it about AD 2100."

Note that this quote mentions that the celestial *pole moves* across
the sky, i.e. across the celestial sphere.  It does not say that the
sky moves across the celestial pole itself being a fixed point on the
celestial sphere.  The celestial sphere, i.e. the set of directions
of the sky w.r.t. the fixed stars, is *just as John claimed*
independent of the coordinatization labeling it.  The coordinate
system does not define the space of directions so labeled.  It is
merely one way to label its independently existing points.  If the
author of the quote *had* actually supported Michael's claim that the
celestial pole defines the celestial sphere (rather than the fixed
stars in it defining the celestial sphere) I would have expected it
to say something to the effect:

 "For example, over the centuries precession causes the sky to move
  across the celestial sphere in such a way that as different stars
  pass near the celestial poles they can, for a while, be used as
  pole stars."

Thus, I fail to see how the quote Michael cites lends any support to
his contention that the celestial poles & celestial equator define
the points of the celestial sphere.  What they *do* do (and what
John and the quote author seem to agree they do) is define a
particular time dependent coordinate system that labels the fixed
points on it in a time dependent manner.

David Bowman

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.